The present invention relates to radiation imaging apparatus and methods and, more particularly, to a novel method and apparatus for ion valve electroradiography, wherein a multiplicity of copies may be secured from a single radiation exposure.
It is well known that radiation, such as X-radiation utilized for medical diagnostic purposes, can be converted into an electrostatic charge image by either a gas, such as Xenon, Krypton, Freon, and the like, under pressure or a radio-conductive liquid, such as tetra-methyl-tin (TMT) and the like. The electrostatic charge image so converted from X-radiation is receivable by a layer of dielectric material for subsequent development into a visible image by conventional xerographic techniques. Apparatus and methods utilizing formation of an electrostatic charge image are described in, e.g. U.S. Pat. No. 3,859,529, issued Jan. 7, 1975; U.S. Pat. No. 3,927,322, issued Dec. 15, 1975; U.S. Pat. No. 3,961,192, issued Jan. 1, 1976; and U.S. Pat. No. 4,064,439, issued Dec. 20, 1977 and assigned to the assignee of the present invention. These exemplary disclosures are incorporated herein by reference.
The sensitivity, in terms of input X-radiation dosage on the x-ray absorber, of these electroradiographic systems is somewhat limited by either x-ray quantum mottle or the minimum developable charge density of commercially available toners utilized to render visible the charge-bearing areas of the insulative film. Generally available commercial toners require an average charge density which exceeds ten nanocoulombs per square centimeter (nC/cm.sup.2). The most sensitive toners, which are not generally available, can develop charge images having an average charge density of 2nC/cm.sup.2. In apparatus having a pair of electrodes enclosing a volume of the radiation-to-charge converting gas or liquid and with the insulative sheet disposed upon the interior surface of that electrode receiving the differentially-absorbed radiation pattern, the resulting radiation dosage required to generate visible images of high quality is greater than an acceptable x-radiation dosage, i.e. an exposure of about 1 milli-Roentgen (mR). Typical sensitivities and dosages can be derived from data published in 1 Medical-Physics 1,262 (A. Fenster et al., 1974) and summarized in the following table:
TABLE ______________________________________ Elec- X-Ray trode Approx. Conversion Spectra Gap Sensitivity Dosage Material (kV.sub.p) (mm.) (nC/cm.sup.2 -mR) (mR.) ______________________________________ TMT 65 2 0.9 11.1 (100kV/cm 80 2 1.2 8.3 field) 100 2 1.9 5.3 65 4 1.0 10.0 80 4 1.3 7.7 100 4 2.2 4.5 XENON 65 10 2.2 4.5 (10 atmospheres) 80 10 2.9 3.4 100 10 3.3 3.0 FREON 13B1 65 10 0.7 14.3 (10 atmospheres) 80 10 0.75 13.3 100 10 0.8 12.5 ______________________________________
It will be seen that visible images require an exposure at least 200% higher than the desired maximum exposure level of 1 mR. In addition, most electroradiographic systems, with the exception of that described in the aforementioned U.S. Pat. No. 4,064,439, can only produce a single radiographic image per radiation exposure, whereby duplication of the original copy must be accomplished by other techniques, such as conventional photocoping using expensive silver-halide film or diazotype prints, with progressively greater defect magnitudes occurring as copies are made of copies, etc. Accordingly, a method and apparatus which cannot only increase the sensitivity of an electroradiographic system, to require an exposure dosage no greater than 1 milli-roentgen, and which can also facilitate generation of multicopies of the radiographic image, as required, is highly desirable.